This invention relates to an improved system for preventing the deflection of a mandrel located inside a mold during thermoplastic injection molding. As used herein, the term "injection molding" includes all types of injection molding and also injection blow molding.
In injection molding and injection blow molding of thermoplastic materials, molten plastic under high pressure is injected into a cavity formed by a mold and a mandrel located inside the mold. This mandrel is rigidly held in place at one end, but is unsupported or free at the other end.
A common problem encountered with this arrangement is elastic deflection of the mandrel. Such elastic deflection is caused by the asymmetrical flow of plastic into the mold cavity, a phenomenon which in practice is unavoidable. The resulting lateral force on the mandrel can lead to substantial deflection, since the general arrangement is that of a relatively slender cantilever beam being subjected to lateral force at the free end thereof. The result is a molding with uneven wall thickness and, in severe cases, only partial filling of the mold cavity.
In conventional injection molding, in order to produce thin structures it is necessary to inject into a very narrow symmetrical area between the mandrel and the mold where the symmetry of the area may be two, three, four, sixfold or of rotational symmetry. Such injection requires excessive injection pressure typically resulting in uneven wall distribution, particularly when a relatively slender configuration is involved.
In conventional injection blow molding or in the case of dual-wall containers, it is desirable to use as small an amount of material as possible for reasons of economy. Because bending forces increase with decreasing parison wall thickness and relative eccentricity of the parison is inversely proportional to the nominal wall thickness for a given mandrel deflection, at present it is necessary to make the wall thickness of the parison 0.05 inch or larger. Using an average blow ratio of three, this results in a wall thickness of 0.166 inch or larger for the finished container. In many container constructions, however, a 0.004 inch wall thickness would be satisfactory, but such a wall thickness cannot be accomplished because of the above-described limitations.
The mandrel deflection problem is most severe when the mandrel is slender, i.e., a high length to diameter ratio, as is the case with most bottles, and when the mold cavity itself is so narrow that very high injection pressures are necessary to fill the cavity with plastic, thus allowing high bending forces to be generated at the free end of the mandrel.
It is, therefore, an object of the present invention to provide a system for preventing the mandrel from deflecting during molding of thermoplastic materials, by pinning or anchoring the mandrel during the initial stage of injection, without interfering with the complete filling of the mold cavity in the final stage of injection.
Molten plastic must at or toward the conclusion of the injection cycle completely surround the free end of the mandrel in order to form a full parison without a hole on the body from the anchoring pin and make this parison suitable for blow molding. To this end, the present invention provides a temporary anchoring during the initial stage of injection when the bending stresses are high, but the anchoring is withdrawn during the final stage of injection when the bending stresses are relieved, thus allowing complete filling of the mold cavity including the point of anchoring.
Another object of the present invention is to provide a mandrel deflection prevention system which is relatively simple in construction and efficient in operation.
Other objects and advantages of the present invention will become apparent as the description proceeds.